Team Fortress 2 is reusing the same contract tracking mechanism from last year's Halloween Event (the Gargoyle) and thus it is showing all the completed Merasmissions from last year as well.
They've also removed the total count from the 'Tome of Merasmissions':
To top it off, new missions keep appearing as I complete them, so I don't even know where I'm up to in general.
The Official Scream Fortress Announcement and TF2 Wiki also do not say how many Merasmissions there are.
How many total Merasmissions are there this year? Is there actually a limit or do they simply continuously spawn?